Lots of reasons. 4K TV's exist and people want to watch these on their modern systems. Zooming in on a vector graphic would produce the same quality image regardless of zoom...that's kinda the point of vector graphics. But that's not what we're talking about here. I think you meant zooming in on a raster image because that would produce the zoom artifacts. But ultimately if they rescanned the film with a higher resolution camera then it's not zooming in on a raster image. You get a much higher fidelity scan so you get 4K without the zoom artifacting.

Essentially if you watch a lower res (say HD or sub HD) movie on a 4K TV, it will look pixelated to some degree. If however you rescan the original film with a 4K camera then watch the resulting file on a 4K TV, you won't have the pixelation.